1. Field of the Invention
The present invention concerns an optical fibre coupler which comprises at least three branches of the optical fibre type. Some of those optical fibres each have a frustoconical end portion fast with one another.
2. Description of the Prior Art
Directional four-branch optical couplers--of which first and third branches are built up from a first optical fibre, and of which second and fourth branches are built up from a second optical fibre--are for example disclosed in the following articles:
article by Takeshi OZEKI and B. S. KAWASAKI in Applied Physics Letters, Vol. 28, No. 9, 1st May 1976, pages 528 and 529, entitled "Optical directional coupler using tapered sections in multimode fibers"; PA1 article by B. S. KAWASAKI and K. O. HILL, in Applied Optics, Vol. 16, No. 7, July 1977, entitled "Low-loss access coupler for multimode optical fiber distribution networks"; and PA1 article by K. O. HILL, B. S. KAWASAKI and D. C. JOHNSON in Applied Physics Letters, Vol. 31, No. 11, 1st December 1977, entitled "Efficient power combiner for multiplexing multiple sources to single-fiber optical systems". PA1 coupling in duplex channels by optical fibres for the purpose of multiplexing optical data signals; PA1 obtaining multifrequency optical channels; PA1 continuous controls exercised on an optical transmission link; PA1 injecting a data or test signal into an optical link or taking such a signal from the optical link.
In disclosed optical couplers, each component branch of an optical fibre is shaped as a frustoconical tapered portion whose large base is prolonged by the straight portion of an end of the fibre and whose small base is amalgamated with that of the frustoconical portion forming the other component branch of the fibre. The two frustoconical portions of a fibre are identical and are obtained by hot softing and pulling under spring tension of the fibre to form a biconical portion. According to usage, the two biconical portions of the first and second optical fibres are joined together by fusion or are separated at their small bases and are embedded in a suitable clear optical adhesive.
For example, the propagation of a light signal from the first branch towards the third and fourth contiguous branches is effected directly in the first fibre and indirectly through at least the coating-to-coating interface of the biconical portions of the first and second fibres. For two given multimode fibres of the same structure which may have different nominal core diameters, the coupling coefficients and directional coefficients between branches are determined by the relative core diameters and by the conicity of the biconical portions of the two fibres. Expressed in other words, the coupling coefficients depend upon the hot pulling of the two fibres, whose characteristics can only be determined approximately insofar as obtaining predetermined coupling coefficients is concerned. Furthermore, for four-branch couplers obtained by predetermined pulling and fusion of the two optical fibres having specified optical and dimensional properties, the coupling coefficients are approximately the same, which does not allow by one and the same manufacturing process to make couplers with different coupling coefficients. Thus after pulling and fusion it is no longer possible to obtain other values of coupling ciefficient from the optical fibres which have been pulled and stuck together. The structure of such couplers does not permit the bidirectional coupling of, on the one hand, an optical fibre to, on the other hand, two other optical fibres without a considerable insertion loss.